Known access mobile networks are so called overlay networks, enabling the setting up of calls between mobile network subscribers and subscribers to other telecommunication networks. Network elements belonging to the mobile network are kept separate from elements belonging to other networks and in consequence of this the networks can be easily managed by different operators. The access element from the mobile network to another telecommunication network, and vice versa, is a mobile switching center MSC, through which not only calls and call-related signalling but also non-call related signalling transit.
Referring to the FIG. 1, in the European GSM network (the Global System for Mobile Communications) a base station sub-system BSS includes a plurality of base transceiver stations BTS for communicating with mobile stations MS through the radio interface and a base station controller BSC. The base station system is connected to the mobile switching center MSC, which can access location registers like a home location register HLR and a visitor location register VLR, the latter one usually being included to the MSC. The location registers are essential elements for mobility management. Using several switching centers connected with trunk lines to each other it is possible to build a mobile subscribers serving network which has large covering area.
In the overlay mobile network, the MSC is connected via SS#7 interfaces to a communication network such as a Public Switched Telephone Network PSTN or Integrated Services Digital Network ISDN and through these networks a mobile subscriber can use services offered by an intelligent network IN.
FIG. 2 depicts the known stack of protocols on the SS#7 interfaces used in communication between networks. Call-related signalling between MSC and external networks makes use of TUP for PSTN, ISUP for ISDN or national variants of these protocols. Non-call related signalling corresponds to many different protocols, which are grouped together in MAP (Mobile Application Part). All MAP protocols use the services provided by the SS#7 protocol TCAP (Transaction Capabilities Application Part), itself using the service offered by the SS#7 protocol SCCP (Signalling Connection Control Part). All these protocols use signalling transport, the lowest layer being referred to as MTP (Message Transfer Part).
Alternative approaches to build up a mobile network are also being studied. It has been proposed to utilize capacity of the ISDN network and an Advanced Intelligent Network AIN, which is specified by Bellcore in his AIN Rel.1 (Advanced Intelligent Network Release 1) and AIN Rel.2. The AIN-network is not a network separated from the ISDN communication network but it utilizes the existing ISDN-network with its transmission lines and exchanges. The physical elements of the AIN include a Service Switching Point SSP, which is a digital exchange being capable to detect calls requiring AIN-services. The services reside in a physical element called Service Control Point SCP, which contains application programs and service logic, whereas service data is stored in data bases called the Service Data Point SDP. The document GR-1299-CORE, `Advanced Intelligent Network (AIN) 0.2 Switch- Service Control point (SCP)/Adjunct Interface Generic Requirements`, published in 1993 by Bellcore in AIN Rel.1, contains Bellcore's view of application and lower layer protocol generic requirements for the interface between the local exchange LE and the SCP. GR-1299.CORE describes messages sent over that interface. The document TR-NWT-001285 `Advanced Intelligent Network (AIN) 0.1 Switch- Service Control point (SCP) Application Protocol Interface Generic Requirements`, published by Bellcore, contains application layer protocol generic requirements for the LE-SCP interface. The application layer protocol is based on the ANSI Transaction Capabilities Application Part (TCAP).
In said alternative approach it has been studied a possibility to drop the MSC of the overlay GSM/DCS1900 network and connect the base station sub-system BSS directly to an ISDN/AIN switch and use its switching capabilities instead of the separate mobile switching center. The result is a radio system shown in FIG. 3. Such radio system is called a C-interface Radio System. The C-interface network architecture is described more detailed in the document ISDN-Based C Interface Access for GSM (DCS1900), Special Report SR-3546, Issue 1, Bellcore, August 1995. C-interface refers to the interface between the radio system and ISDN-network. The ISDN-based C-interface is the most important interface in this mobile network. At present, regarding mobility management no solutions and no standards exist for the C-interface but at least two types of transport options have been suggested: in the first type the radio system has access to the SS#7-signalling network so being capable to communicate with the SCP and an overlay network element directly through the SS#7 network. This first type is named later in this document as RS/SS#7-type. In the second type the radio system has no access to the SS#7 signalling network but it can communicate with the SCP using non-call associated signalling NCAS as access protocol. This second type is called RS/NCAS type later in this document. In both types, all call associated signalling i.e. call control signalling between the radio system and the local exchange LE uses as the access protocol ISDN's DSS1 (Digital Subscriber Signalling System No. 1), which is described in ANSI (American National Standard) T1.602-1989 (Layer 1 specification) and T1607-1990 (Layer 3 specification). Signalling between the local exchange LE and the SCP uses the known AIN application protocol. In the C-interface radio system the application programs in AIN Service Control Point (SCP) are used to provide the mobility management functions.
In comparison to the system shown in FIG. 1, the block RS (Radio System) in FIG. 3 includes not only some of the functions of the traditional BSC but also a part of the functions of the visitor location register VLR and the mobile switching center MSC. It takes care of many of the MSC responsibilities like hand-over related switching. The local exchange LE handles basic call process and may, according the above-mentioned second type of the radio system, support non-call associated (NCAS) signalling. It is Standard National ISDN 3 LE or some later version. SCP has ability to communicate with MAP added so it can do locating queries to HLR and locating queries to VLR. The functions of VLR are divided between SCP and RS.
In the overlay mobile network a protocol for managing the mobility of subscribers is called Mobile Application Part MAP. Also in the C-interface system a mobility protocol shall be specified between each of the radio systems RS and between each radio system RS and the service control point SCP. This protocol called Mobility Management Protocol (MMAP) is being standardized by ANSI/T1 subcommittee T1S1. The MMAP for PCS1900 will be based on MAP.
A problem in modifying an overlay network architecture to fit to the ISDN/AIN architecture is caused by the lack of MSC. For example, the known GSM network is designed to fit to the ISDN-network by means of the MSC. Another problem is caused by the lack of means for realising mobility management between the C-interface radio systems without MSC and the traditional overlay mobile network with MSC. This can be reduced to the problem how to create a mechanism for exchanging information between C-interface radio system and the MSC or another element belonging to the overlay network.
In FIG. 4 is depicted a straightforward way to solve the problem. All the RSs, MSC/VLR and HLR are connected to the global SS#7 network. Thus the RSs may be considered similar to MSC/VLR. This case may be seen equivalent to GSM case. When communicating towards an overlay network element like MSC, the C-interface radio system uses SS#7 signalling and those same protocols used in the overlay network. Also the address information like Global Titles, Subsystem Numbers, etc., it sends is similar to that of the overlay network element. This means that the hand-off and roaming operations transferred via SS#7 between two RSs and between the radio system RS and overlay network elements are standardized MAP operations or corresponding MMAP operations. Due to this the anchor RS does not need to know whether the target is MSC/VLR or another RS. Additionally, some C-interface specific MMAP protocol operations not having counterparts in the traditional MAP protocol may be defined between the radio system RS and the SCP. Also the SCP uses the MAP protocol when communicating with the HLR.
Further, SCP is not involved when the hand-off procedure is performed between network elements in SS#7 network. The MAP connections to SCP are presented here only to allow interworking with RS/NCAS entities. The hand-off and roaming procedures in this solution are considered obvious and therefore signalling flow diagrams are not presented.